Method for developing cavities in salt or other soluble rock



April 2, 1957 R. s. KNAPPEN METHOD FOR DEVELOPING CAVITIES IN SALT OROTHER SOLUBLE ROCK Filed March 29, 1955 INVENTOR. QUSSEZL 5. xowpp/w MILFOR DEVELOPING CAVTTIES IN SALT OR OTHER SOLUBLE RGCK 8 Claims. (Ci.262-3) METHOD This invention relates to a method of forming undergroundreservoirs for the storage of gases or liquids, particularly petroleumproducts, such, for example, as liquefied petroleum gas. The inventionhas particular reference to a method for forming underground storagecavities in soluble-rock formations by dissolving the rock with asuitable solvent.

It is known that, generally speaking, underground storage reservoirsshould preferably be of roughly conical form for the purpose ofproviding a structurally strong reservoir and one in which theoverburden will prevent loss of materials stored under high pressure. Aconical form also provides the strongest roof as a protection againstroof collapse. In other instances, other shapes, e. g., a hollow sphere,may bepreferred.

It is an object of the present invention to provide a method of formingshaped reservoirs in soluble-rock formations for the storage of liquidsor gases.

Briefly stated, my invention comprises the method of forming a shapedunderground reservoir in which a well hole is drilled into asoluble-rock formation, casing is set in the hole down to a pointbetween a substantial distance above, and a substantial distance below,the top of the soluble-rock formation, e. g., usually about ten butpossibly several hundred feet, the casing is cemented, at least oneoutlet and one inlet tubing, preferably concentric, are lowered into thehole and to a depth below the casing seat, non-dissolving sealing liquidis forced down through the space between the casing and the containedtubes to below the casing seat, a solvent that is immiscible with andheavier than the sealing liquid and which will dissolve rock of theformation is then pumped down the inlet tubing, and, after dissolvingrock of the formation is withdrawn or forced out through the outlettubing as a solution of the material forming the soluble-rock formation.Dissolving or leaching is continued until a cavity of predetermined sizeis produced. The tubing is then lowered in the borehole into positionfor solution of another section of the activity. Sealing liquid is thenpumped into the first section of the cavity preferably through the spacebetween the tubes and the casing, until the immiscible liquid almostcompletely displaces the solvent in this cavity, above the depth of theshorter of the inlet or outlet tubing. This solvent is forced back upthe outlet tube. Introduction of solvent is resumed, and solution of asecond section of the cavity begins. This section of the cavity can bemade larger in area than the first section of the cavity by continuingthe dissolving until a larger amount of dissolved rock is carried to thesurface. In similar manner this section of the cavity can be smallerthan the preceding section of the cavity by dissolving the formation fora shorter time before introducing sealing liquid to displace the solventand prevent further dissolution of the walls and roof of the cavity.

A saturated solution of solvent is substantially higher in specificgravity than the more dilute solvent being introduced into the cavityand saturated solvent will tend to lie on the floor of the cavity andprotect it from further solution. In other words, there will be sometendency for the solution to stratify in the cavity being formed andsolution will proceed primarily in a lateral direction or at least morerapidly in a horizontal than a vertical direction thus making itpossible to form cavities of vast area. This effect is considerablyincreased if the solvent outlet tubing is extended to a level much lowerthan the solvent inlet tubing.

It is a feature of my invention so to dispose the relative positions ofthe open ends of the inlet and outlet tubing in order to govern theshape of the cavity being formed, a lower outlet tubing forming cavitieswith larger horizontal dimensions and a lower inlet tubing cavities withgreater vertical dimensions. The open ends of the inlet and outlettubing are preferably maintained at least 10 feet apart.

Although reservoirs can be dissolved out of formations such as dolomiteor limestone by using an acidic aqueous solution according to the methodof my invention, I especially contemplate forming underground reservoirsin rock salt formations or salt domes in which the solvent employed canbe fresh water or a dilute salt water and the sealing liquid an oil.

In the accompanying drawings which serve to illustrate a preferredembodiment of my invention,

Fig. 1 illustrates a borehole, sealed casing, inlet and outlet tubing,and associated apparatus for forming shaped reservoirs, and

Fig. 2 is an elevational cross sectional view showing a partiallycompleted reservoir, the remainder of the reservoir being shown indotted outline.

Referring specifically to Fig. l, to illustrate a specific example ofdrilling a well and forming a shaped reservoir, a borehole 10 is drilledby the usual rotary drilling equipment (not shown) down to cap rockwhich is reached at 870 feet and thence through the cap rock formation11 into a salt formation 12 which is reached at a depth of about 2100feet. Drilling is continued for another 1800 feet into the saltformation. A casing string 13 is then set into the well to a depth whichextends into the salt formation 12.

The casing is fixed in position by forcing cement down through casing 13and up through the annular space between casing 13 and borehole it).Drilling fluid in the hole below the end of the casing preventsimportant downward movement of cement, which must rise toward thesurface. A plug is pumped down the casing to force most of the cementout of the casing. Pumping is stopped before the plug is forced out ofthe casing. After the cement sets, the plug in the casing is drilledout. Two concentric strings of tubing, 14 and 15, are then lowered intothe well. The outer tubing 14 is lowered to a depth below the casingshoe 17 and the inner string of tubing 15 is lowered to a depth of about200 feet below the bottom of the outer tubing 14. The above depthdifferential of 200 feet is selected to form the first and uppermostsection 19 of a conical cavity. Four repeated settings and dissolvingsteps are required to form the entire cavity, as shown in Fig. 2. A sealis formed by introducing an oil, or other water-immiscible liquid havingspecific gravity less than water, into the annulus 13 between the outertubing string and the casing 13. This liquid is pumped down the annulus18 and will flow out of the bottom of the casing and will rise up intoany annular space between the casing 13 and the borehole 10, which wasnot filled by cement. The oil or water-immiscible liquid is pumped intothe cavity to displace any water which stands in the hole above thebottom of the outer tubing string 14. This oil will prevent anysubsequent solution of salt above the lower level of the oil orwater-immiscible liquid.

The importance of the seal of water-immiscible liquid between the openhole and the casing and the outside string of tubing was strikinglydemonstrated in one instance where no seal was employed in leaching outa cavity with an unsaturated brine. The dissolving of salt around thecasing created a cavity having a volume of more than 400,000 gallons.The hydrocarbons which subsequently occupied this cavity when thereservoir was put in use could not be recovered except by perforatingthe casing and, in addition, this uncontrolled leaching left a longlength of unsupported casing extending into the cavity.

About 5,000 barrels per day of fresh or slightly salty water are pumpedinto the hole through the outer string of tubing 14 and are returned,except for that amount of solution which remains to displace dissolvedsalt, to the surface for disposal or sale. At this rate of leaching withfresh water, a 50,000 barrel reservoir can theoretically be formed inabout sixty days. After allowance for interruptions and inefficiency andrate of solution, a 50,000 barrel cavity should be formed in 150 days.When the uppermost section 19 of the cavity has been substantiallycompletely formed as shown in Fig. 1, the tubing strings 14 and 15 arethen lowered into the borehole 10 and oil is pumped down through theouter annulus 18 until the brine which had occupied the cavity 19 isdisplaced upwardly through the tubing string 15. A seal is again formedby the oil and leaching proceeds beneath the seal in the same manner asoccurred in the leaching of the cavity section 19. This process isrepeated until four sections are formed as shown in Fig. 2. Any desirednumber of sections can be removed to form a cavity of the size and shapedesired.

Fig. 2 is an elevational view of the entire cavity showing thesuccessively formed sections 19, 19a, 19b, and 190. Section 190 isincompletely formed, the process of solution of the rock not having beenentirely completed. During the time that section 190 is being formed, anoil seal extending down through and filling sections 19, 19a, and 19bprevents further solution by the water of these upper sections of thecavity. The tendency of dense, saturated brine to settle at the bottomof the reservoir forms a protective layer 21 which prevents furtherleaching downwardly. Outer tubing string 14 serves as an inlet pipe forsolvent and concentric inner tubing string 15 for the flow of brine tothe surface. After leaching has continued in the section 190 until thecavity has been increased in size to approximately the dimensionsindicated by dotted lines 22 and 23, additional oil is pumped by thepump 24 down through the annular space between the casing 13 and thetubing string 14 until the brine in section 19c has been substantiallycompletely forced out of the cavity. Brine removed from the cavity 190is pumped by pump 25 to pit 26 and to conventional means (not shown) fordisposal.

Circulation of the leaching fluids can be reversed so as to introducedilute solvent through inner string 15, and withdraw saturated solutionthrough the annular space between the tubing strings 14 and 15.

When the leaching process is completed the tubing strings 14- and 15 canbe pulled and a string of large tubing (not shown) of, for example,about /2 inches in outside diameter can be run in and suspended orbottomed at substantially total depth in the reservoir shown in Fig. 2.A frame or derrick or pulling mast varying from 65 feet to 140 feet inheight and a pulling machine are employed for running, adjust-ing, andpulling the above mentioned tubing strings.

In forming a shaped reservoir by the continuous method, the outer tubingstring (inlet string) is lowered gradually down the well hole. The ratesof dissolving and of lowering (and relative position, as aforesaid, ofthe inlet and outlet tubing) determine the lateral dimension of thecavity provided by leaching, wider cavities being formed when theinjection of solvent is more rapid and when tubing string moves moreslowly. As the outer tubing string is moved progressively downwardly theinner tubing string may be lowered at substantially the same rate andthe seal of solvent-immiscible liquid is pumped into the well at a ratesuch that the interface between the sealing liquid and the leachingsolvent will travel downwardly at substantially the same rate as that ofthe tubing. It is necessary that the bottom open hole of the outertubing remain at all times below the said interface. An oil seal canconsist of a hydrocarbon that is to be stored in the well so as to avoidhaving to pump out the sealing liquid after the cavity has been formed.

When the cavity has been formed and filled with stored fluid, this fluidcan be withdrawn from storage by various methods. If the stored fluid isa gas, or will become a gas on relief of pressure, the fluid may bewithdrawn by merely reducing pressure at the casing head, which is thetop of the casing cemented in the overlying formation. If the fluid isdesired in liquid form, but will gasify upon release of pressure, thefluid may be recovered by injecting saturated brine through tubing 15extending to a point near the bottom of the cavity and forcing thestored fluid under pressure to the surface through tubing 14 or throughcasing 13, or both. If the stored fiuid will not gasify upon release ofpressure, it may be pumped by ordinary oil-field-pumping methods throughtubing 15. Alternatively, the fluid may be forced to flow to the surfacethrough tubing 15 by injecting a gas under suflicient pressure into theupper part of the cavity through tubing 14 or through casing 13, orboth. Alternatively, saturated brine may be injected into the bottom ofthe cavity through tubing 15, thereby forcing the lighter stored fluidto rise to the surface through tubing 14 or casing 13, or both.Saturated brine must be used in cavities in salt to avoid furthersolution of the cavity walls and roof, with resulting destruction of thepreferred conical or other formed shape of the cavity. The positions ofthe lower ends of tubing strings 14 and 15 will normally be adjusted tosecure the results desired with a minimum of agitation of the storedfluid and a minimum of mixing of injected and withdrawn materials.

My method and the reservoirs formed thereby are especially well adaptedto the storage of hydrocarbons or gases under high pressures, sinceamong other shapes, conical shapes can be produced which will providethe strongest possible roof shape for maintaining the cavity intact andresisting the pressure of fluids stored in the cavity.

Having described my invention, I claim:

1. A method of forming shaped underground reservoirs in soluble-rockformations, the said method comprising: drilling a well through anoverburden and into a soluble-rock formation; cementing casing in thewell, the casing seat being located at a point between a substantialdistance above, and a substantial distance below, the top of thesoluble-rock formation; introducing a tubing string into the cased welland running it to a substantial depth below the open bottom end of thesaid casing; running another tubing string into the well to an evengreater depth; pumping a sealing liquid, that is relatively anon-solvent with respect to the soluble rock formation, into the welland permitting the sealing liquid to flow into open hole below thecasing and rise up around thecasing; pumping a solvent that isimmiscible with and heavier than the sealing liquid down through one ofthe said tubing strings; introducing the solvent into the well and thusbringing it into contact with the soluble-rock formation and dissolvingrock of the said formation; forcing a solution of the rock in thesolvent upwardly to the surface through the other of the said tubingstrings; after a period in which a cavity of a predetermined capacityand approximate lateral dimension has been excavated by said dissolving,introducing additional sealing liquid into the well and thus loweringthe seal that is formed by the sealing liquid; also lowering the tubingstrings in the well and repeating the dissolution process until another,lower cavity of a predetermined capacity and approximate lateraldimension has been formed; and continuing a repetition of the foregoingsteps while varying the period of dissolving to obtain cavities ofpredetermined approximate lateral dimension until a shaped reservoid hasbeen formed.

2. In a method of forming shaped underground reservoirs in soluble-rockformations, in which a borehole is drilled ultimately to total reservoirdepth, casing is cemented in the borehole to a depth which includes somefluid-impermeable rock, and tubing strings are run in, one to a pepth inthe soluble-rock formation below the casing, and another to a greaterdepth, the steps of: pumping a non-solvent sealing liquid downwardlythrough space between the casing and tubing strings to and beyond thebottom of the casing; pumping a solvent that is heavier than andimmiscible with the sealing liquid down through one tubing string intocontact with the soluble-rock formation and thus dissolving rock of theformation; forcing solvent containing dissolved rock upwardly to thesurface through another tubing string; after a selected period in whicha cavity of a predetermined capacity and approximate lateral dimensionhas been so excavated, introducing additional sealing liquid into thewell thus lowering the seal formed thereby; lowering the tubing stringsand repeating the dissolution process at a lower point in the well; andcontinuing a repetition of the foregoing steps until a shaped reservoirhas been formed.

3. A method of forming underground reservoirs in soluble-rock formationsfor storage therein of fluids, the said method comprising: drilling awell through overburden and an impermeable rock formation into solublerock formation; setting casing into the impermeable rock formation to adepth within the said impermeable formation; cementing the casing in theso-formed open hole; running at least two strings of tubing into thecased well, one to a depth slightly below the bottom of the casing, andanother to a substantial depth below the bottom of the other tubing;pumping a sealing liquid that is immiscible with and of lower densitythan solvent used in leaching the soluble-rock formation into the welland to and beyond the bottom open end of the casing; pumping solventdown through one tubing and into dissolving contact with soluble rock inthe open well; by said pumping of solvent, forcing solvent and dissolvedrock upward through a second tubing to the ground surface; after aperiod in which a cavity of a predetermined capacity and approximatelateral dimension has been so excavated, introducing additional sealingliquid into the well and thus lowering the seal that is formed by thesealing liquid; also lowering the tubing strings in the well andrepeating the dissolution process until another, lower cavity of apredetermined capacity and approximate lateral dimension has beenformed; and continuing a repetition of the foregoing steps, whilevarying the period of dissolution to obtain cavities of predeterminedapproximate lateral dimension, until a shaped reservoir has been formed.

4. A method of continuously forming shaped underground reservoirs inwater-soluble salt formations, the said method comprising: drilling awell ultimately to total reservoir depth; setting casing into theresulting borehole and cementing the same; introducing tubing stringinto the cased well and running it to a substantial depth below thebottom open end of the said casing; running another tubing string intothe well to an even greater depth; pumping an oil into the annular spacebetween the well and the tubing strings; lowering the tubing strings,while introducing water through one of the strings and removing water,containing dissolved salt, through another, at a rate of advance such aswill provide a lateral cavity of approximate predetermined dimension;and continuing to introduce oil at a rate to form an interface travelingdownwardly at substantially the rate of lowering the tubing strings toseal from further dissolving action the surfaces of the already formedreservoir.

5. The method of claim 4 in which the tubing strings are concentric,water is pumped into the well through the outer, shorter tubing andwater containing dissolved salt is removed through the inner, longertubing.

6. A method for forming underground reservoirs of generally conicalshape in water-soluble salt formations comprising drilling a boreholeinto the salt formation; cementing a casing in the borehole to preventupward movement of fluids outside of the casing; running at least twostrings of tubing down through the easing into the borehole below thelower end of the casing, one of the strings of tubing being longer thanthe other and extending deeper into the borehole than the other; pumpingwater down one of the strings of tubing into the salt formation todissolve the salt and force brine formed by dissolution of the salt upthe other string of tubing; pumping a water-immiscible sealing fluidhaving substantially no solvent efiect on salt and a density lower thanwater down the casing into the borehole and cavity formed by dissolutionof the salt formation to form a water-immiscible fluid-brine interfaceabove the ends of the strings of tubing; removing brine containing apredetermined amount of salt from the salt formation; lowering thestrings of tubing to a deeper position in the salt formation; pumpingadditional water-immiscible fluid into the borehole and cavity to lowerthe water-immiscible fluid-brine interface substantially the samedistance the strings of tubing were lowered; and repeating the steps ofpumping water into the formation, to remove a predetermined amount ofsalt, lowering the tubing and lowering the level of the water-immisciblefluid brine interface; the amount of salt removed per foot of loweringof the strings of tubing being greater as the tubing is lowered deeperinto the salt formation.

7. A process as set forth in claim 6 in which water is discharged fromthe shorter string of tubing into the salt formation and brine isdelivered from the salt formation through the longer string of tubing.

8. A method for forming underground reservoirs of generally conicalshape in water-soluble salt formations comprising drilling a boreholeinto the salt formation; cementing a casing in the borehole to preventupward movement of fluids outside of the casing; running at least twostrings of tubing down through the easing into the borehole below thelower end of the casing, one of the strings of tubing being longer thanthe other and extending deeper into the borehole than the other; pumpingwater down one of the strings of tubing into the salt formation todissolve the salt and force brine formed by dissolution of the salt upthe other string of tubing; pumping a water-immiscible sealing fluidhaving a density lower than water down the easing into the borehole andcavity form-ed by dissolution of the salt formation to form awater-immiscible fluid brine interface above the ends of the strings oftubing; removing brine containing a predetermined amount of salt fromthe salt formation; lowering the strings of tubing to a deeper positionin the salt formation; pumping a volume of water-immiscible fluidsubstantially equal to the volume of salt removed in the precedingdissolving step into the cavity in the salt formation to lower thewater-immiscible fluid brine interface substantially the same distancethe strings of 7 tubing-we're loweredg-and' repeating fhe steps ofpumping] water into the formation, to remove a predetermined amount of'salt, lowering the tubing and lowering the level of thewater-immiscible'fluid" brineinterface; the amount of salt removedper foot" of loweringof the 5 stringsof tubing being greaterasthe tubingis' lowered deeperintothe salt formation;

References Cited in the' file of this' gatent UNITED STATES PATENTSFOREIGN PATENT S France Aug. 13'', 19.06

1. A METHOD OF FORMING SHAPED UNDERGROUND RESERVOIRS IN SOLUBLE-ROCKFORMATIONS, THE SAID METHOD COMPRISING: DRILLING A WELL THROUGH ANOVERBURDEN AND INTO A SOLUBLE-ROCK FORMATION; CEMENTING CASTING IN THEWELL, THE CASING SEAT BEING LOCATED AT A POINT BETWEEN A SUBSTANTIALDISTANCE ABOVE, AND A SUBSTANTIAL DISTANCE BELOW. THE TOP OF THESOLUBLE-ROCK FORMATION; INTRODUCING A TUBING STRING INTO THE CASED WELLAND RUNNING IT TO A SUBSTANTIAL DEPTH BELOW THE OPEN BOTTOM END OF THESAID CASING; RUNNING ANOTHER TUBING STRING INTO THE WELL TO AN EVENGREATER DEPTH; PUMPING A SEALING LIQUID, THAT IS RELATIVELY ANON-SOLVENT WITH RESPECT TO THE SOLUBLE ROCK FORMATION, INTO THE WELLAND PERMITTING THE SEALING LIQUID TO FLOW INTO OPEN HOLE BELOW THECASING AND RISE UP AROUND THE CASTING; PUMPING A SOLVENT THAT ISIMMISCIBLE WITH AND HEAVIER THAN THE SEALING LIQUID DOWN THROUGH. ONE OFTHE SAID TUBING STRINGS; INTRODUCING THE SOLVENT INTO THE WELL AND THUSBRINGING IT INTO CONTACT WITH THE SOLUBLE-ROCK FORMATION AND DISSOLVINGROCK OF THE SAID FORMATION; FORCING A SOLUTION OF THE ROCK IN THESOLVENT UPWARDLY TO THE SURFACE THROUGH THE OTHER OF THE SAID TUBINGSTRINGS; AFTER A PERIOD IN WHICH A CAVITY OF A PREDETERMINED CAPACITYAND APPROXIMATE LATERAL DIMENSION HAS BEEN EXCAVATED BY SAID DISSOLVING.INTRODUCING ADDITIONAL SEALING LIQUID INTO THE WELL AND THUS LOWERINGTHE SEAL THAT IS FORMED BY THE SEALING LIQUID; ALSO LOWERING THE TUBINGSTRINGS IN THE WELL AND REPEATING THE DISSOLUTION PROCESS UNTIL ANOTHER,LOWER CAVITY OF A PREDETERMINED CAPACITY AND APPROXIMATE LATERALDIMENSION HAS BEEN FORMED; AND CONTINUING A REPETITION OF THE FOREGOINGSTEPS WHILE VARYING THE PERIOD OF DISSOLVING TO OBTAIN CAVITIES OFPREDETERMINED APPROXIMATE LATERAL DIMENSION UNTIL A SHAPED RESERVOID HASBEEN FORMED.